The major objective of the experiments proposed in this application is to further understand the hormonal regulation of Ca and P homeostasis during reproduction. These two elements, besides being major constituents of the skeleton, play critical roles in numerous physiological processes and control of both their concentrations in body fluids and their fluxes in various tissues are essential to the normal functioning of the organism. The following experiments were designed to be performed in rats. (1) Determine if vitamin D influences prolactin secretion by comparing plasma prolactin levels in vitamin D-deficient and -replete rats at various stages of reproduction. (2) Measure renal 25-OH-D3-1-hydroxylase activity in pups during the weaning period to determine the requirement for 1,25-(OH)2D3 to stimulate intestinal Ca transport and bone mineral mobilization. (3) Measure Ca and P balances throughout lactation to determine the relative contributions of maternal diet and skeletal reserves as sources of Ca and P in milk. (4) Examine the role of vitamin D-independent mechanisms in the stimulation of intestinal Ca transport during lactation by measuring intestinal hypertrophy and Ca balances in vitamin D-deficient rats.